Selective IgA deficiency (IgAD, 1/600) and common variable immune deficiency (CVID, 1/25,000) are the most common forms of human primary immunodeficiency in Caucasians. Individuals with symptomatic IgAD/CVID suffer from recurrent infectious complications, and are prone to autoimmune disorders and lymphomas. To date, mutations in three genes, CD19, ICOS and TACI were found in a limited subset of IgAD/CVID patients making up approximately 10% of the affected individuals. A recent report by our group identified a significant association of genetic variation in the DNA mismatch repair (MMR) gene MSH5 with IgAD/CVID. The MSH5 gene is located in the MHC class III region, the genetic locus linked with IgAD/CVID in most patients. We found that one of the MSH5 alleles identified contained two nonsynonymous polymorphisms (L85F/P786S), and the variant protein encoded by this allele showed impaired binding to MSH4, a heterodimeric partner of MSH5. IgS?-S?1 switch joints from IgAD and CVID patients carrying disease-associated MSH5 alleles showed increased donor/acceptor microhomology, involving pentameric DNA repeat sequences and lower mutation rates than controls. H-2b/b congenic MRL/lpr mice carrying a hypomorphic allele for Msh5 exhibit a profound deficiency of IgG3 and limited IgA deficiency in over 70% of mice. We found increased switch joint microhomology at S?-S?3 and S?-S? in IgG3-deficient H-2b/b MRL/lpr, Msh5-/- and Msh4-/- mice similar to IgAD/CVID patients carrying disease-associated MSH5 alleles. These results suggest that both Msh5 and Msh4 play a role in Ig class switch recombination (CSR), similar to other MMR proteins that are known to be involved in both CSR and somatic hypermutations (SHM). Recently, we found that Msh5-/- mice had significantly decreased frequency of SHM and increased number of dC/dG single nucleotide deletions in the Ig JH4 gene compared to wild-type mice. Based on these findings, we hypothesize that Msh5 has a previously unrecognized significant role in both CSR and SHM in both mice and humans with deficiency of Msh5 leading to immune deficiency. We also hypothesize that single nucleotide deletions at coding regions in Ig V(D)J rearrangements in Msh5-/- mice cause frameshifts that result in altered Ig protein production and decreased serum Ig levels, the latter a common feature in CVID patients. To further define the role of Msh5 and its mechanisms of action in IgAD/CVID, we propose the following Specific Aims: (1) Further evaluate the role of Msh5 and Msh4 in SHM and immune deficiency by analyzing SHM and serum Ig levels in aged or immunized Msh5-/-, Msh4-/-, and control mice. (2) Determine the mechanism/pathway of Msh5 effect on SHM/CVID using double gene-knockout mice for other known MMRs. (3) Define the association and localization of Msh5 with other MMRs during induced in vitro SHM by using immunoprecipitation and Chip assays. These experiments will provide further understanding of the role of Msh4/Msh5 in CSR, SHM and IgAD/CVID in humans and perhaps new approaches to therapy. PUBLIC HEALTH RELEVANCE: IgAD and CVID are the most common innate human immunodeficiencies affecting up to 1/600 Caucasians. Individuals suffering from these diseases experience increased rates of infection, autoimmunity and lymphoid malignancies. This line of investigation will provide new insight into the molecular basis of these immune deficiencies through novel mouse models that will rapidly translate into human disease.